Italy's centre-left government was facing an outcry yesterday over law and order after figures emerged linking a wave of gangland murders to a mass pardon rushed through parliament soon after it took office.

In an effort to defuse the crisis the prime minister, Romano Prodi, visited Naples, where 12 killings in 10 days - several carried out on crowded streets - prompted calls to send in the army.

Mr Prodi dismissed the idea, saying "an increase in police would be more efficient". Giuliano Amato, the interior minister, had earlier pledged to increase police numbers by 1,000.

Nine prisoners freed as a result of the pardon have been involved in killings in and around Naples over the past two months. In some cases they were the alleged murderers, but in others they were the victims.

According to the latest figures, more than 24,000 convicted offenders have been freed under a measure approved by parliament at the end of July. The pardon was intended to ease overcrowding in Italy's jails.

Recent polls have suggested that the measure is among the most unpopular actions taken by the government.

The pardon appears to have had a dramatic effect in mafia-ridden areas such as Naples and the surrounding region of Campania where more than 2,700 prisoners have been freed.

The return to their "turfs" of hundreds of former mobsters since August has, in many cases, upset delicate power balances between rival clans.

One of the victims in the last 10 days was murdered apparently as a reprisal for attempting to leave the Neapolitan mafia, known as the Camorra. Italian mafiosi swear an oath of allegiance at the start of their criminal careers that only lapses when they die. The four most recent victims in Naples were all killed in archetypal Camorra-style ambushes.

The daily La Repubblica quoted Franco Roberti, head of the anti-Camorra division of the Naples prosecution service, as saying: "There is an objective fact: that former prisoners let out because of the pardon have played a role in almost all the homicides of the past three months."

Touring Naples yesterday, Mr Prodi said there was "no statistical link, no connection, between the pardon and the crime rate". But some of his own supporters are unconvinced. Antonio Di Pietro, a former prosecutor who leads a small anti-corruption party, said the pardon had created a "sense of growing impunity".

The measure was also enacted before steps could be taken to find jobs or, in some cases, homes for those released. Mr Di Pietro said the result was that many petty criminals who had been let out were "wandering the streets with no other option than to go back to crime".

Naples, once among the most beautiful cities of the Mediterranean, suffers from both rampant street crime and the depredations of the Camorra. There are almost 100 muggings and robberies a day, while the activities of the region's mobsters are estimated to have an annual turnover of €16bn (£11bn), largely from protection, construction and the management of the waste disposal business.

But Naples is not the only southern city to have been hit by an upsurge in violence. At Bari, on the other side of the peninsula, where a barman was shot dead on Halloween by an armed robber wearing a skeleton mask, the centre-left mayor, Michele Emiliano, said on Thursday: "The pardon is the key issue ... in Bari we had the lowest of crime rates. But now things have changed."

Murder in Naples

September 11 Giacomo Selva, fresh out of jail, arrested for ordering a gangland hit in the Camorra-ridden Secondigliano district

September 17 Four young men are charged with stabbing to death a town hall employee. Two were beneficiaries of the pardon

September 18 Another freed prisoner arrested for taking part in a fatal city centre shoot-out in which a Canadian tourist was wounded

October 22 Salvatore Attanasio, would-be Camorra renegade, murdered

October 31 Two more men set free by the pardon are shot dead in Torre del Greco, near Naples